(1) Field of the Invention
The present invention relates to a process for the preparation of a glazed ceramic substrate. More particularly, the present invention relates to a process for the preparation of a glazed ceramic substrate on which fine electric wiring is possible. Furthermore, the present invention relates to a glazing composition for use in carrying out this process and a paste for use in coating this glazing composition.
(2) Description of the Prior Art
A glazed ceramic substrate comprising a glazing layer of a glass melt-bonded to the surface of a ceramic substrate is widely used as a substrate on which a filmy electric wiring is formed, because the surface of this substrate is excellent in the smoothness.
This conventional glazed ceramic substrate is ordinarily prepared by coating a paste comprising a glass powder composed of SiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and the like, an appropriate solvent and an appropriate binder on the surface of a ceramic substrate composed of alumina, beryllia or the like according to the screen printing method and sintering the ceramic substrate to melt-bond the glazing paste to the ceramic substrate. A ceramic substrate having a metallized metal portion of tungsten (W), molybdenum (Mo) or the like on the surface is prepared by coating the glazing paste only on the surface of the ceramic substrate exclusive of the metallized metal portion.
However, in the case where a glazing layer is melt-bonded to a ceramic substrate having this metallized metal portion, in order to coat a glazing paste only on the surface of the ceramic substrate exclusive of the metallized metal portion, a special mask should be used for preventing the glazing paste from adhering to the metallized metal portion. Since this mask is expensive and registering of the mask with the ceramic substrate is difficult, the productivity of the glazed ceramic substrate is low and the manufacturing cost of the glazed ceramic substrate is increased.
Furthermore, the glazing paste is poor in the printability and bleeding is readily caused. Accordingly, when the glazing paste is coated and printed only in the portion exclusive of the metallized metal portion on the ceramic substrate, a certain clearance should be formed from the metallized metal portion while taking bleeding of the glazing paste into consideration.
However, bleeding of the glazing paste is irregular, and if bleeding of the glazing paste is smaller than expected, the glazing paste cannot be melt-bonded to the portion close to the periphery of the metallized metal portion on the surface of the ceramic substrate, and if a fine electric wiring is distributed in this glazing layer-free portion, breaking of the wiring is caused because of the surface roughness of the ceramic substrate and the function of the fine electric wiring substrate is completely lost.